1. Field of the Invention
This invention relates to the field of carbon molecular sieve membranes. More particularly, it relates to carbon molecular sieve membranes showing high selectivity in ethylene/ethane separations.
2. Background of the Art
Ethylene is one of the largest volume organic chemicals produced globally. Frequently produced commercially from petroleum and natural gas feedstocks, much of the production cost unfortunately goes into separation of ethylene (C2H4) from ethane (C2H6). Currently, C2H4/C2H6 separation is carried out almost exclusively by cryogenic distillation, which is an extremely energy-intensive process because of the relatively low relative volatility differential between C2H4 and C2H6 (1.75). A typical distillation may require a temperature of −25 degrees Celsius (° C.) and a pressure of 320 pounds per square inch gauge (psig) (approximately (˜) 2.21 megapascals (MPa)). As a result, very large distillation towers, employing in some cases from 120 to 180 trays and high reflux ratios, are required, making it a very expensive separation.
Membrane technology provides an attractive alternative to such thermally driven separations, because it may require less energy and reduce environmental impact. Membranes are widely used for separation of liquids and gases. Gas transport through such membranes is commonly modeled by a sorption-diffusion mechanism, wherein gas molecules sorb at the upstream face of the membrane, diffuse through the membrane under a chemical potential gradient, and finally desorb at the downstream side of the membrane. Two intrinsic properties are used to evaluate the separation performance of a membrane material: its “permeability,” a measure of the membrane's intrinsic productivity; and its “selectivity,” a measure of the membrane's separation efficiency. “Permeability” is typically measured in Barrer, which is calculated as the flux (ni) divided by the partial pressure difference between the membrane upstream and downstream (Δpi), and multiplied by the thickness of the membrane (l).
      P    i    =                    n        i            ⁢                          ⁢      l              Δ      ⁢                          ⁢              p        i            
Another term, “permeance,” is defined herein as the productivity of asymmetric hollow fiber membranes and is typically measured in Gas Permeation Units (GPU), which are calculated by dividing the permeability in Barrer by the membrane thickness in microns (μm)
      (                  P        i            l        )    =            n      i              Δ      ⁢                          ⁢              p        i            
Finally, “selectivity” is defined herein as the ability of one gas's permeability or permeance in comparison to the same property of another gas, to pass through the membrane. It is measured as a unitless ratio.
      ∝          i      /      j        ⁢      =                            P          i                          P          j                    =                        (                                    P              i                        /            l                    )                          (                                    P              j                        /            l                    )                    
Currently, polymers are the dominant membrane material used for gas separations because of their processability and selectivity for a variety of gas separations in general. The performance of these polymeric membranes is often, however, limited by an upper bound trade-off curve between productivity (permeability) and efficiency (selectivity). In addition polymeric membranes may be inadequate for high pressure applications of sorptive gases, for example, hydrocarbons, since they may undergo plasticization, which may result in significant loss in performance. Plasticization may be a particularly serious problem for asymmetric hollow fiber configurations.
In contrast, carbon molecular sieve (CMS) membranes have been discovered to be both robust and stable for certain high pressure applications (up to 1,000 pounds per square inch (psi), ˜6.89 MPa), often with better separation performance than that of polymeric membranes for many gas separations. CMS membranes are typically produced by pyrolysis of polymer precursors under controlled conditions. For example, it is known that hollow fiber CMS membranes can be produced by pyrolyzing cellulose hollow fibers. In addition, many other polymers have been used to produce CMS membranes. Certain polyimide polymers have been found especially useful because of their high glass transition temperatures, desirable processability, and rigidity following pyrolysis.
For example, U.S. Pat. No. 6,565,631 describes a method of synthesizing a CMS membrane by pyrolyzing a commercial polyimide hollow fiber precursor in an evacuated environment following a ramp-soak temperature protocol to produce a high carbon content filamentary membrane. This membrane is described as being useful to separate carbon dioxide (CO2) from a mixed stream of natural gas. For additional examples of production of high carbon content filamentary membranes, the reader may wish to also review U.S. Pat. Nos. 5,288,304 and 4,685,940 and EP Patent 459,623.
Another CMS membrane prepared from a polyimide is described in P. J. Williams, Carbon Molecular Sieves for Ethane-Ethylene Separation based on 6FDA and BPDA Polyimides, AIChE Talk, Nov. 2004. In that case, the starting precursor material is an exotic polyimide that is not commercially available and the CMS membranes are synthesized exclusively in dense film configurations and under vacuum only.
Research has shown that CMS membrane properties are affected by the following primary factors: (1) pyrolysis precursor, (2) precursor pretreatment conditions, (3) pyrolysis temperature, ramp rate and thermal soak time, (4) pyrolysis atmosphere and (5) post-treatment conditions. The effect of these factors on CMS performance for a variety of gas separations has been investigated by several researchers, but to date a stable, reproducible CMS membrane, readily and economically prepared from commercially available materials, configurable as both dense films and hollow fibers, and offering improved performance in C2H4/C2H6 separations, has not been identified.